The present invention relates to an optical semiconductor device and a method for fabricating the same, more specifically an optical semiconductor device having a ridge waveguide structure and a method for fabricating the same.
As semiconductor lasers for the high speed optical communication are known the buried-heterostructure (BH) and the ridge structure.
The buried-heterostructure includes an active layer buried in a material of different refractive index to thereby confine light in the active layer by the refractive index difference. The buried-heterostructure can have low threshold oscillation and single-transverse-mode by confining current and light in micronized regions in both the vertical and the transverse directions.
On the other hand, the ridge structure has a simple structure including a mesa stripe of a clad layer formed on an active layer, and the structure for narrowing the current and light can be formed by a simple fabrication method. In comparison with the buried-heterostructure, the ridge structure has a merit that the fabrication cost can be lower.
The conventional optical semiconductor device having the ridge structure will be explained with reference to FIG. 11.
On a substrate 100, there are formed a lower clad layer 102, a lower light confinement layer 104, an active layer 106, an upper light confinement layer 108, an upper clad layer 110 and a contact layer 112 are formed. The contact layer 112 and the upper clad layer 110 are processed in a mesa stripe. The mesa stripe does not reach the upper light confinement layer 108. The mesa is formed down to a lower part of the upper clad layer 110 without arriving at the upper clad layer 110. A passivation film 114 is formed on the side surface of the mesa stripe and the upper surface of the clad layer 110. Electrodes 116, 118 are formed respectively on the backside of the substrate 10 and on the contact layer 112.
The related arts are disclosed in, e.g., Reference 1 (Japanese published unexamined patent application No. Hei 02-178989), Reference 2 (Japanese published unexamined patent application No. Hei 10-178200), Reference 3 (Japanese published unexamined patent application No. Hei 08-255932), and Reference 4 (Japanese published unexamined patent application No. 2001-127333).
To realize a direct modulation light source of low cost, the ridge structure is prospective as described above. However, the ridge structure has found it difficult to concurrently realize the parasitic capacitance decrease and the suppression of higher-order transverse mode oscillation.
In the optical semiconductor device of the ridge structure shown in FIG. 11, the width of the mesa stripe is set at about 2 μm, whereby the higher-order transverse oscillation can be suppressed. However, the parasitic capacitance is very large because of the p-i-n junction formed between the upper clad layer 110 and the lower clad layer 102 which is extended all over the element. Accordingly, the frequency band determined by a CR product of the device is so narrow that the high-speed modulation is made impossible.
In order to decrease the parasitic capacitance of the device, as shown in FIG. 12, it is effective to completely etch off the upper clad layer 110 to form a mesa stripe down to the upper light confinement layer 108. However, this structure makes the light confinement too strong, and the oscillation tends to take place in the higher-order transverse mode.
With the mesa stripe covered with an electrode material when the electrodes are formed, the leaked light is absorbed by the electrode material, and the oscillation threshold value often increases.